Cervantes, I don't doubt you get great image resolution with a 5D3. However, if you do some simple math, you will find that something like a 7D resolves detail within its cropped area, equal to that of a 46 MP (or so) full frame...which exceeds that of a D800, let alone a 5D3. My own 15.1 MP crop camera, resolves detail within its cropped 1.6x field, of a 38.5 MP full frame. So, just by the math, and guessing, I'd say mine resolves a similar amount of detail to a D800, and perhaps a bit less than a D800E (not discussing dynamic range, etc.)
The 1D Mark 4, has a pixel size which is equal to approximately 25MP on a full frame, within its 1.3x crop area.
So there is more detail there for a 1.6x...gobs of it (provided the ISO is reasonably low of course).
In a recent review, I saw where the detail falls off sharply on the 6D above ISO 6400, where it doesn't fall off until above ISO 12,800 for the 5D3. Interesting.
The ~24MP 1.6x crop sensors of the future, will resolve so much detail that I highly doubt anyone will ever be able to produce files with a 100% crop, where the lens will resolve as much as the pixels will resolve. I've seen a similar problem with the full size images from the Sigma SD1, which can theoretically resolve anywhere from 30 to 40 MP, via its 1.5x crop sensor. That's a full frame equivalent of well over 100 MP.
There are other factors at play, and I suggest your experience with a 550D, is at least in part due to a lack of AFMA...if not a very wimpy autofocus sensor...or both. The processing comes into play also.
That's the problem with going from a Rebel to a 5D3. You missed out on the best part of the crop camera experience!
If you're arguing about switching to a longer lens for a similar field of view on full frame, then that's apples and oranges...but yes, a longer lens on a full frame camera with larger pixels, will mean the lens itself need not be as sharp, as a shorter lens needs to be, in order to make use of a crop camera's smaller pixels...to produce a similar amount of detail for a similar subject size, over the same pixel area of the competing files.
However, there is a law of diminishing returns. The rare, expensive, and old Canon 1200mm lens on a 5D3, will probably not resolve what the newer, less expensive, 800 f/5.6 will do, mounted to a 7D. I could be wrong, but certainly from what I have seen online, the 1200mm isn't all that sharp at all (even manually focused on a stationary target, which was Manhattan in the shot I'm remembering, I think.)
So that gets back to my point. If you need a long telephoto in low light, then the best choice would always be a full frame, or otherwise whatever has the lowest noise sensor and the best autofocus (and likely it would have the larger area for each photosite, diode, or pixel...and the larger autofocus sensor). If you need a long telephoto for shooting unusually fast or agile targets...you probably need to couple it to a 1Dx. If they aren't moving very fast, you can probably get by with a 7D on an even shorter lens. I'm not stating anything new there, I don't think.
